Several new classes of herbicides, such as the sulfonylureas, have inhibition of acetolactate synthase as their apparent mode of action. Acetolactate synthase and acetohydroxyacid synthase are two names for the enzyme EC 4.1.3.18, which will hereinafter be referred to as ALS/AHAS. ALS/AHAS catalyzes conversion of pyruvate to acetolactate in the first step of a multistep biosynthetic pathway by which plants synthesize valine and leucine. ALS/AHAS inhibitors are of great interest as herbicides for a variety of reasons. Because vertibrates do not synthesize valine and leucine, the mode of action does not threaten vertibrates. Further, many of the newly discovered compounds are active at very low doses.
The emergence of target-site based resistance to ALS/AHAS inhibiting herbicides has, however, raised concern for the viability of this class of products. The onset of resistance has been both rapid and extensive. A total of eleven weed species resistant to ALS/AHAS inhibiting herbicides are known. These are distributed over at least four countries, eleven U.S. states, and three Canadian provinces. Well over 400 sites of Kochia resistance have been identified in North America during the past six years. Three consecutive years of sulfonylurea use in Idaho wheat prodution selected for resistance in Lactuca serriola. Further, resistant biotypes do not appear to suffer any significant penalty in terms of fitness. The relatively rapid emergence of resistance, coupled to high fitness of resistant biotypes, necessitates the development of resistance management strategies.
Resistance management efforts will have a significantly greater chance of succeeding if a method is available to rapidly identify resistant weeds. Ideally, a field researcher should be able to sample tissue from a putative resistant weed, test for resistance either in the field or back in the laboratory, and plan and execute an appropriate strategy all within a few hours.